As is known in the art, there exists a relatively large number of commercially available devices having a base or stationary portion and a movable cover or door portion which include a magnet. For example, telephones, cellular telephones, notebook or laptop computers and refrigerators include magnets in the moveable door or cover portions. The covers are typically opened and closed and, in some cases, the magnets provide a magnetic force which maintains the cover or door in a particular position (e.g. a closed position).
Such devices can also include detectors or sensors which indicate when a door or cover is in an open or a closed position. For example, cellular telephones (cell phones) which are provided as so-called “flip phones,” include a base and a cover or “flip” portion. The cover has a magnet disposed therein. Disposed in the base portion of the cell phone is a sensor. When the cover is closed, the magnet is disposed over the sensor and the sensor detects the presence of the magnet's magnetic field. In response to the magnetic field, the sensor provides a signal which indicates that the cover is closed. Similarly, when the cover is open, the magnet (and hence the magnetic field) is removed from the sensor and the sensor provides a signal indicating that the cover is open.
In some applications, the sensor is provided as a Reed switch. The Reed switch is a mechanical type switch comprised of an evacuated glass tube having a series of metal fingers disposed therein. In response to the presence a magnetic field, the metal fingers are in mechanical contact thus providing a signal path having a short circuit impedance characteristic between the input and output terminals of the switch. Likewise, in the absence of a magnetic field, the mechanical fingers are not in contact thus providing a signal path having an open circuit impedance characteristic between the input and output terminals of the switch.
Reed switches have the advantage that the switch operates regardless of the orientation of the magnet with respect to the switch. That is the Reed switch need not be oriented in a particular manner with respect to the poles of the magnet. This allows for easy replacement of the magnet or the Reed switch since there is not physical relationship between them.
One problem with the Reed switch approach, however, is that the Reed switch is relatively large and expensive when compared with semi-conductor type switches. Also, the Reed switch is a mechanical type switch and thus is not as reliable as a solid state devices.